1. Field of the Invention
This invention relates to increasing the brightness of an arc lamp by folding the arc back into itself.
2. Description of the Related Art
U.S. patent application Ser. No. 09/604,921, the disclosure of which is incorporated by reference, describes a dual-paraboloid reflector system that may be used to couple light from an arc lamp into a target such as a standard waveguide, e.g., a single fiber or fiber bundle, or output electromagnetic radiation to the homogenizer of a projector. This optical collection and condensing system, as illustrated in FIG. 1, uses two generally symmetric paraboloid reflectors 10, 11 that are positioned so that light reflected from the first reflector 10 is received in a corresponding section of the second reflector 11. In particular, light emitted from a light source 12, such as an arc lamp, is collected by the first parabolic reflector 10 and collimated along the optical axis toward the second reflector 11. The second reflector 11 receives the collimated beam of light and focuses this light at the target 13 positioned at the focal point.
The optical system of FIG. 1 may employ a retro-reflector 14 in conjunction with the first paraboloid reflector 10 to capture radiation emitted by the light source 12 in a direction away from the first paraboloid reflector 10 and reflect the captured radiation back through the light source 12. In particular, the retro-reflector 14 has a generally spherical shape with a focus located substantially near the light source 12 (i.e., at the focal point of the first paraboloid reflector) toward the first paraboloid reflector to thereby increase the intensity of the collimated rays reflected therefrom.
U.S. application Ser. No. 09/669,841, the disclosure of which is incorporated by reference, describes a dual ellipsoidal reflector system that may be used to couple light from an arc lamp into a target. This optical collection and condensing system, as illustrated in FIG. 2, uses two generally symmetric ellipsoid reflectors 20, 21 that are positioned so that light reflected from the first reflector 20 is received in a corresponding section of the second reflector 21. In particular, light emitted from the light source 22 is collected by the first elliptical reflector 20 and collimated along the optical axis 25 toward the second reflector 21. The second reflector 21 receives the collimated beam of light and focuses this light at the target 23 positioned at the focal point.
The objective of the above-described systems that collect, condense, and couple electromagnetic radiation into a target is to maximize the brightness of the electromagnetic radiation at the target. These systems must be efficient and have relatively long useful lives.
Arc lamps, e.g., metal halide lamps, xenon lamps, or high pressure mercury lamps, are often used in the above-mentioned systems as sources of light. One of the means by which high brightness may be obtained is by making the arc gap in the lamp small such that all the light is emitted from a small spot. An ideal source is a point source, in which the distance between the electrodes is negligible. There are practical limitations, however, to reducing the distance between the electrodes below a certain value. Among the limitations associated with a shorter arc are a loss of emission efficiency and reduced electrode life. The useful lives of the electrodes will be shorter with the shorter arc.
Since arc lamp gaps cannot be reduced indefinitely, there remains a need to increase the brightness of the electromagnetic radiation emitted by arc lamps with longer gaps for coupling into a target.